AD 97-23-06 Amendment 39-10194

This amendment adopts a new airworthiness directive (AD), applicable to Schweizer Aircraft Corporation Model 269A, A-1, B, and C, and TH-55A helicopters, with a certain main rotor transmission ring gear (ring gear) installed, that requires inspections of the ring gear teeth for surface deterioration which includes pitting, excessive wearing, cracking or corrosion, and replacement of the ring gear if such ring gear teeth surface deterioration is found; and also requires creating a main rotor transmission component log card (log card), if none is available, and making a notation on the log card if a ring gear is changed. This amendment is prompted by reports of failures of the ring gear due to single tooth distress as a result of improper gear tooth spacing during the manufacturing of the ring gear. The actions specified by this AD are intended to prevent failure of the ring gear, loss of drive to the main rotor gearbox, and a subsequent forced landing.